In recent years, there are a lot of applications in which an optical film is stuck and fixed onto a transparent plastic substrate (for example, polycarbonate-made substrates and acrylic resin-made substrates) as a substrate using a pressure-sensitive adhesive or a pressure-sensitive adhesive tape or sheet. In such applications, the pressure-sensitive adhesive or pressure-sensitive adhesive tape or sheet is required to have not only transparency such that visibility of a transparent plastic substrate or optical film is not hindered but also lifting and peeling preventing properties (resistance to foaming and peeling) such that after weathering tests under severe conditions such as high-temperature conditions and high-temperature and high-humidity conditions, lifting or peeling at the adhesion interface due to air bubbles (foams) generated from the plastic substrate or the like is not caused. Concretely, under high-temperature or high-temperature and high-humidity conditions, plastic substrates such as acrylic resin-made substrates and polycarbonate-made substrate may possibly cause outgassing that is a cause of foaming considered to be caused by the absorbed moisture or residual monomer. For this reason, if a weathering test is performed by sticking an optical film made of a base material film having barrier properties, such as polyethylene terephthalate (PET) films, on an acrylic resin-made substrate or polycarbonate-made substrate using a pressure-sensitive adhesive or pressure-sensitive adhesive tape or sheet, lifting or peeling occurs at the adhesion interface with the pressure-sensitive adhesive layer due to air bubbles (foams) generated from the substrate or the like, whereby not only the appearance lowers, but also the visibility is markedly hindered.
Thus, in these applications, there have hitherto been made attempts to prevent lifting or peeling from occurrence even if air bubbles are generated from an adherend such as substrates, by, for example, properly adjusting the molecular weight of an acrylic polymer as a base polymer of the pressure-sensitive adhesive, adding various additives for increasing adhesion to the plastic substrate, such as a tackifier, to the pressure-sensitive adhesive, or copolymerizing the polymer formulation of a base polymer of the pressure-sensitive adhesive with a monomer component capable of increasing adhesion (see Patent Documents 1 to 8).
[Patent Document 1]
JP-A-1-178567
[Patent Document 2]
JP-A-2002-30264
[Patent Document 3]
JP-A-2001-335767
[Patent Document 4]
JP-A-2001-89731
[Patent Document 5]
JP-A-2000-109771
[Patent Document 6]
JP-A-2003-49143
[Patent Document 7]
JP-A-2002-327160
[Patent Document 8]
JP-A-10-310754
(The term “JP-A” as used herein means an “unexamined published Japanese patent application”)
However, in the method of properly adjusting the molecular weight of an acrylic polymer as a base polymer of the pressure-sensitive adhesive, there are limitations in improving the lifting and peeling preventing properties (resistance to foaming and peeling). Also, in the method of adding a tackifier to the pressure-sensitive adhesive, since if the compounding amount of the tackifier is large, transparency is hindered, the compounding amount is obviously limited. Accordingly, in this case, there are limitations in improving the lifting and peeling preventing properties, too. Further, with respect to the polymer formulation for increasing adhesion, its lifting and peeling preventing properties were limited so that high requirement levels could not be satisfied.